What are the differences between 同時に and 一斉に?

Question

I understand that 一斉に{いっせいに} and 同時に{どうじに} are adverbs and they both mean "at the same time, simultaneously", so I am wondering if there is any nuance or difference between them ?

I encountered the former in a newspaper article, so I am also wondering if there is some politeness attached to it ?

Answer 1

This may not be correct, but my impression is that with 一斉, it's multiple "copies" of an action happening at the same time such that it really only feels like one action.

• 一斉メール　→　Sending one email to multiple recipients at the same time
• 選挙の開票は全国一斉に行われる　→　Ballot counting will begin simultaneously [at the same time] throughout the country.
  • So even though it's clearly many different people/groups doing the counting all over the country, the "counting" still feels like just one action.
• ベルが鳴ると皆一斉に立ち上った　→　The moment the bell rang, they all stood up at once.
  • Again, even though it's multiple students standing up, it seem like only one joint action.

Whereas 同時 doesn't necessarily/usually have this nuance, and the multiple "actions" feel (more) distinct and separate.

• 同時通訳　→　Simultaneous interpretation
• 山登りは楽しいけど、同時にとても危ない　→　Mountain climbing is fun, but at the same time very dangerous.
• 彼が入ってきた、それと同時にベルが鳴った　→　He came in, and at the same time the bell rang.
  • Two distinct things happening at the same time

Though there does seem to be a good amount of overlap.

Answer 2

I am wondering if there is any nuance or difference between them ?

Yes, there is. I'll try to explain my opinion.

I am also wondering if there is some politeness attached to it ?

No, politeness isn't the factor

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一斉に

• I would say "along with"
• Timing is not important compared to 同時に
• Larger scale compared to 同時に
• Dynamic and impactful compared to 同時に

Examples

• At a concert, 一斉に拍手 round of applause
• At a marathon, 一斉に走り出した start running at once

It's used when a phenomenon occurs "along with" each other. It's not really important that we start clapping our hands at 10:00:01, is it?

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同時に

• I would say "simultaneously"
• Timing is important compared to 一斉に. I say compared to 一斉に, because 同時に is not always used for timing-critical stuff
• Not necessarily a large-scale or group activity, can be used for small boring things (press button simultaneously)

Answer 3

The word "一斉" is used when a lot of people do something at the same time.
e.g.

• コンサート会場{かいじょう}に居{い}た人々{ひとびと}が一斉{いっせい}に拍手{はくしゅ}をした。
  The people in the concert hall clapped in unison.

On the other hand, "同時" is used when (not a lot of) people do something at the same time.
e.g.

• 路上{ろじょう}パフォーマンスを見{み}ていた人々{ひとびと}が、同時{どうじ}に拍手{はくしゅ}をした。
  People who were watching the street performance clapped at the same time.

So, we use different words depending on the number of people.